1. Field of the Invention
The present invention relates to a process for improving the thermal stability of a resin composition comprising a polyurethane resin and a polyvinyl chloride resin, and the composition thereby obtained. More particularly, the invention relates to a process for improving the thermal stability of polyurethane-containing resin compositions, characterized in that a sulfur-containing carboxylic acid metal salt having the formula: ##STR2## wherein R.sup.1 is cycloalkyl, alkenyl or aralkyl group or alkyl having 4 to 18 carbon atoms,
R.sup.2 and R.sup.3 each are hydrogen, methyl, ethyl or aryl, and PA1 M is barium, calcium or magnesium, PA1 R.sup.2 and R.sup.3, which can be the same or different, are hydrogen, methyl, ethyl or aryl, wherein aryl preferably has 6 to 12 carbon atoms, most preferably benzyl, and PA1 M is barium, calcium or magnesium, preferably calcium and barium, most preferably barium.
Is incorporated in a resin composition comprising a polyurethane resin and a polyvinyl chloride resin blended at a weight ratio of from 9:1 to 3:7.
2. Description of the Prior Art
In general, polyurethane resins possess excellent properties such as abrasion resistance, solvent resistance and toughness, but because they have urethane linkages, namely, carbonamide linkages ##STR3## they are inferior in weatherability and thermal stability. Accordingly, when they are exposed to heat during molding, for example, injection molding, extrusion molding or calender molding, they are drastically degraded and discolored. In addition, polyurethane resins are disadvantageous because they are more expensive than other thermoplastic resins, such as polyvinyl chloride, polyethylene and the like, and the ease of processing the same during molding is insufficient.
If the foregoing disadvantages of polyurethane resins are overcome, it will be possible to use them more effectively in various fields for the production of films, sheets, artificial leathers, belts, tubes, hoses, packings, coated wires and other electric appliances, as powder adhesives for sewing and as adhesives for making shoes by injection molding. Therefore, it has been desired in the art to overcome the foregoing disadvantages of polyurethane resins.
It has been considered that these defects can be overcome by blending polyurethane resins with relatively inexpensive common thermoplastic resins having an excellent processability during molding, particularly polyvinyl chloride resins, and various attempts to do so have heretofore been made. However, a heat stabilizer having a good effect of preventing thermal degradation of such a composition has not been found and, therefore, a satisfactory improvement has not been attained in the art.
We investigated the effects of improving the thermal stability of blends of polyurethane resins and polyvinyl chloride resins by the use of known stabilizers for polyurethane resins such as nitrogen-containing stabilizers (for example, phenyl-.beta.-naphthylamine), sulfur-containing stabilizers (for example, .beta.-laurylthiopropionic acid), phenol type stabilizers (for example, 2,6-di-tert-butyl-4-methylphenol) and phosphorus-containing stabilizers (Irganox 1890 (product of Ciba-Geigy) and trinonylphenyl phosphite), and known stabilizers customarily used for polyvinyl chloride resins such as calcium stearate, zinc stearate, barium stearate, cadmium stearate, cadmium-barium type composite metal soaps, organotin compounds, organic phosphorous acid esters, epoxy compounds and antioxidants, and we used these stabilizers singly and in combination. However, contrary to our expectation, a satisfactory result could not be obtained in each case.
It was found that a sufficient thermal stabilization effect cannot be attained by the use, in resin compositions comprising a polyurethane resin and a polyvinyl chloride resin, of either or both of the thermal stabilizers known for use in polyurethane resins and in polyvinyl chloride resins, and that a quite different kind of thermal stabilizer must be developed for such resin compositions.
Organotin compounds which have an excellent thermal stabilizing effect to polyvinyl chloride resins are insufficient in the effect of improving the thermal stability of resin compositions comprising a polyurethane resin and a polyvinyl chloride resin, but it has been found that barium stearate which is inferior in the initial thermal stabilizing effect to polyvinyl chloride resins has a relatively high effect of improving the thermal stability of such resin compositions.